transfer signal via wire question

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Hi there,

I would like to send a signal via a wire about 30-40 meters length (max). The signal is produced by an AVR PWM output (10bit, ~20Khz, 0-5V) and it contains audio data. I can send the PWM signal as it has or pass it from a sallen-key active filter before send it.

I would like to ask a question about the advantages and the disadvantages of transfering the signal. Which method is better for my case ?

- PWM signal as it has or
- AUDIO signal (after the sallen-key filter)

If the better sollution is the second one, then what kind of audio output is better DC (as it has) or AC (using a capacitor in series) ?

Thank you.

Michael.

User of:
IAR Embedded Workbench C/C++ Compiler
Altium Designer

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I would use an RS422 transmitter at one end and an RS422 receiver at the other. This would immunize the signal, to a very great degree, from noise and other signals. Then, filter at the far end.

By the way, to make a practical filter, you need the PWM repetition frequency to be on the order of 10X of the highest frequency component of the modulation. That is, if you want the highest audio frequency to be 5KHz, then the PWM ought to be at least 50KHz repetition frequency. This criterion is the same, no matter which end of the line contains the filter.

Jim

 

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It depends if the signal on the wire should be rather analog audio or digital bitstreams, and what would be the bandwidth of audio. I assume about 4kHz BW or 8kHz sampling rate. So sure, 4kHz audio bandwith is easier to put through than digital pulses that resemble 8kHz sampling rate. As said, PWM rate should be far higher, and basically the PWM rate does not mean signal bandwith, as you can have one narrow PWM pulse with length of 1 count. Digital pulse has infinite bandwith, and the system must guarantee you can transmit that PWM pulse without much distorsion.

If cabling is say CAT5 cable, it can carry audio, PWM or other digital signals, and frankly, the RS422 option will crave for twisted pair anyway.

This would sound like if RS422 is used, why use it for PWM, as you could transmit UART data between devices, 8-bit PCM audio, just select baud rate that is slightly higher than audio data rate, so that every now and then there is more than one byte worth of silence so any disturbance that happens gets a chance to resynchronize.

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With such a long wire you are likely to have issues with transmission line effects if you are sending a signal with high frequency components - as is the case with digital signals - ie PWM.

The fundamental frequency of the signal doesn't matter for a transmission line, but rather the ratio of the signal's ride or fall time compared to the time it takes that signal to traverse the transmission line (2/3 speed of light is a rough guide). ie 20KHz is not particularly high, but the edges of your PWM signals could have ries times in the nanosecond range, so the frequency components of that part of the signal are very high indeed.

If I were you I would probably look to send the signal as a low bandwidth analogue signal. If you wish to do it digitally then ensure that you use a cable with a known impedance and ensure you use suitable drivers and receivers and termination.

Learning about transmission lines will stand you in good stead for the future when even a few inches on a PCB can become a transmission line and a signal integrity challenge.

-Tim